The present invention relates generally to the manufacture of negative cadmium electrodes for sealed alkaline battery cells, in strip form, which are subjected to a partial oxidation to form a charge reserve by a chemical treatment with oxidizing agents outside the cell.
It is known that in alkaline battery cells operating under sealed conditions (e.g., of the Ni/Cd type) it is not sufficient to design the two electrode polarities with equal capacities, since a degree of overload protection should also be present in addition to the required rated capacity of the cell, and to avoid pole reversal during discharge with high currents. Thus, the negative electrode capacity in a cell designed for stable operation (i.e. a well "balanced" cell) is always overdimensioned in comparison to the positive electrode so that the negative electrode has a useful capacity component corresponding to the positive electrode capacity, and an excess capacity which is divided between a charge reserve and an electrode reserve. In the fully discharged state of the cell, the negative electrode is discharged only partially, such that dischargeable Cd.sub.met is still present in the amount of the available discharge reserve. In the fully charged state of the cell, the negative electrode is charged only partially, as distinguished from the positive electrode, and maintains this state even during continued charging. This is because the negative charging current which continues to flow does not reduce further Cd(OH).sub.2 to Cd.sub.met, but only converts the oxygen developed in equivalent quantity on the positive electrode. The negative cadmium electrode thus remains partially oxidized to the extent of the intended charge reserve.
A proven method which has long been used in the manufacture of sealed batteries involves bringing completely assembled, but still open cells to the above-mentioned fully charged state by electrical treatment, whereupon the cells are sealed as rapidly as possible. However, in view of the expensive equipment needed, and the processing time and manpower required, attempts have been made to balance the electrodes prior to being installed and to prepare the electrodes chemically or electrochemically in such a way that ready-to-use cells are immediately obtained after the electrodes have been introduced into the container, the electrolyte has been added, and the cell has been sealed. Depending on whether the process is based on uncharged or charged negative electrode material, the state necessary for this is reached either by chemical or electrochemical partial oxidation, or by partial reduction.
West German Patent Application No. P 34 16 817 suggests the use of hydrogen peroxide as a highly advantageous agent for the partial oxidation of a negative electrode strip (so-called electrodeposit electrode), which is already highly active due to its processing. After washing and drying, a partially oxidized negative strip is obtained which is sufficiently stable with regard to its capacity. Cut into electrodes, this strip can be combined with positive electrodes which are adjusted accordingly with respect to their total capacity and their state of charge. A cell built with electrodes pretreated in this manner can be sealed immediately after the introduction of the necessary amount of electrolyte (usually a KOH solution), and can be put into operation immediately without any expensive electrical treatments.
Introduction of the electrolyte into the already fully assembled, but as yet unsealed cell is automatically carried out in conventional manner, e.g. by means of reciprocating pumps at very high production rates. However, in this process, the evolution of heat, which is dependent on cell size, has been found to be disturbing. Such evolution of heat is caused by decomposition of the heavy metal peroxides which are spontaneously formed in the alkaline medium by the desorption of oxygen, as well as by the heat of hydration released during the wetting of the still dry electrodes. Even small cells can reach temperatures of up to 70.degree. C. within a few seconds, and still higher temperatures are reached in the larger cells.
Bubbling caused by boiling and sputtering of the electrolyte tends to reduce the accuracy of metering, and the high temperatures are harmful in themselves because they tend to accelerate degradation of polyamide separators, and to cause changes in the pre-established electrode balance.